Objective Guidelines for Removing an External Fixator after Tibial Lengthening Using Pixel Value Ratio: A Pilot Study

Abstract

During limb lengthening over an intramedullary nail, decisions regarding external fixator removal and weightbearing depend on the amount of callus seen at the lengthening area on radiographs. However, this method is subjective and objective evaluation of the amount of callus likely would minimize nail or interlocking screw breakage and refracture after fixator removal. We asked how many cortices with full corticalization of the newly formed bone at the lengthening area are needed to allow fixator removal and full weightbearing and how to radiographically determine the stage of corticalization. We retrospectively reviewed 17 patients (34 lengthenings) who underwent bilateral tibial lengthenings over an intramedullary nail. The average gain in length was 7.2 ± 3.4 cm. We determined the pixel value ratio (ratio of pixel value of regenerate versus the mean pixel value of adjacent bone) of the lengthened area on radiographs. There were no nail or screw breakage and refracture. Partial weightbearing with crutches was permitted when the pixel value ratio was 1 in two cortices and full weightbearing without crutches was permitted when the pixel value ratio was 1 in three cortices. The pixel value ratio on radiographs can be an objective parameter for callus measurement and may provide guidelines for the timing of external fixator removal. We cannot determine from our limited data the minimum pixel value in how many cortices would suggest safe removal, but we can say our criteria were not associated with subsequent refracture.

S-osteotomy with lengthening and then nailing compared with traditional Ilizarov method

“…The external fixation index was defined as the time in external fixation in days divided by the length gained in centimetres. Consolidation was considered to be complete when anteroposterior and lateral radiographs confirmed at least three of four cortices were intact [13]. The consolidation were measured by two of the authors.…”

Comparison between upper and lower limb lengthening in patients with achondroplasia

“…However, we feel that this may not be significant, as it was validated in the senior author's previous study. 13,19 In conclusion, humeral lengthening is a safe procedure with a relatively lower complication rate than in the femur. The callus formation in the humerus during the distraction period proceeds at a significantly higher rate than in the femur.…”

“…The PVR has recently been validated as an objective evaluation of mineralisation in the lengthening zone. [19][20][21] It is a cheap, cost-effective and sensitive parameter for measuring callus stiffness. However, to our knowledge, no previous study has used serial PVR measurement and callus pattern analysis in cases of humeral lengthening.…”

Computational modelling of ovine critical-sized tibial defects with implanted scaffolds and prediction of the safety of fixator removal

“…is the ratio of the mean callus pixel value to the mean intact bone pixel value, and is the cross-sectional moment of inertia of each individual defect. Using the suggested threshold of = 1 for the safe removal of external fixation [21], none of the four defects studied here qualify for removal of external fixation. It is interesting to note that threshold values of each measure ( and ) that have been calculated to predict the safe removal of fixation are not significantly greater than the predicted values for defects with β-TCP scaffolds (0.89 and 0.86).…”

“…The most common clinically-used criterion for the removal of fixation is the measurement of the thickness of continuous cortices of bone bridging a defect using X-rays, with reported refracture rates of 4% [18] and 3% [19]. A pixel value ratio ( ), evaluated from X-ray images, has also been used as a guide for fixation removal, with re-fractures reported in defects with < 0.8 [20] and a suggested optimal value of 1 [21]. The 2D nature of these quantitative callus assessment methods is a limiting factor as they do not account for 3D variations in callus density or continuous loading paths of bone.…”